Sample inspection system

ABSTRACT

A curved mirrored surface ( 78 ) is used to collect radiation scattered by a sample surface ( 76   a ) and originating from a normal illumination beam ( 70 ) and an oblique illumination beam ( 90 ). The collected radiation is focused to a detector ( 80 ). Scattered radiation originating from the normal and oblique illumination beams may be distinguished by employing radiation at two different wavelengths, by intentionally introducing an offset between the spots illuminated by the two beams or by switching the normal and oblique illumination beams ( 70, 90 ) on and off alternately. Beam position error caused by change in sample height may be corrected by detecting specular reflection of an oblique illumination beam and changing the direction of illumination in response thereto. Butterfly-shaped spatial filters may be used in conjunction with curved mirror radiation collectors ( 78 ) to restrict detection to certain azimuthal angles.

This application is a continuation of application Ser. No. 08/993,771,filed Sep. 19, 1997, now U.S. Pat. No. 6,201,601.

BACKGROUND OF THE INVENTION

This invention relates in general to sample inspection systems and, inparticular, to an improved inspection system with good sensitivity forparticles as well as crystal-originated-particles (COPs). COPs aresurface breaking defects in semiconductor wafers which have beenclassified as particles due to inability of conventional inspectionsystems to distinguish them from real particles.

Systems for inspecting unpatterned wafers or bare wafers have beenproposed. See for example, PCT patent application Ser. No.PCT/US96/15354, filed on Sep. 25, 1996, entitled “Improved System forSurface Inspection.” Systems such as those described in theabove-referenced application are useful for many applications, includingthe inspection of bare or unpatterned semiconductor wafers.Nevertheless, it may be desirable to provide improved sample inspectiontools which may be used for inspecting not only bare or unpatternedwafers but also rough films. Another issue which has great significancein wafer inspection is that of COPs. These are surface-breaking defectsin the wafer. According to some opinions in the wafer inspectioncommunity, such defects can cause potential detriments to theperformance of semiconductor chips made from wafers with such defects.It is, therefore, desirable to provide an improved sample inspectionsystem capable of detecting COPs and distinguishing COPs from particles.

SUMMARY OF THE INVENTION

This invention is based on the observation that anomaly detectionemploying an oblique illumination beam is much more sensitive toparticles than to COPs, whereas in anomaly detection employing anillumination beam normal to the surface, the difference in sensitivityto surface particles and COPs is not as pronounced. Anomaly detectionemploying both an oblique illumination beam and a normal illuminationbeam can then be used to distinguish between particles and COPs.

One aspect of the invention is directed towards an optical system fordetecting anomalies of a sample, comprising first means for directing afirst beam of radiation along a first path onto a surface of the sample;second means for directing a second beam of radiation along a secondpath onto a surface of the sample and a first detector. The systemfurther comprises means including a mirrored surface for receivingscattered radiation from the sample surface and originating from thefirst and second beams and for focusing the scattered radiation to saidfirst detector.

Another aspect of the invention is directed towards an optical systemfor detecting anomalies of a sample, comprising first means fordirecting a first beam of radiation along a first path onto a surface ofa sample; second means for directing a second beam of radiation along asecond path onto a surface of the sample, said first and second beamsproducing respectively a first and a second illuminated spot on thesample surface, said first and second illuminated spots separated by anoffset. The system further comprises a detector and means for receivingscattered radiation from the first and second illuminated spots and forfocusing the scattered radiation to said detector.

One more aspect of the invention is directed towards an optical systemfor detecting anomalies of a sample, comprising a source supplying abeam of radiation at a first and a second wavelength; and means forconverting the radiation beam supplied by the source into a first beamat a first wavelength along a first path and a second beam at a secondwavelength along a second path onto a surface of a sample. The systemfurther comprises a first detector detecting radiation at the firstwavelength and a second detector detecting radiation at the secondwavelength; and means for receiving scattered radiation from the samplesurface and originating from the first and second beams and for focusingthe scattered radiation to said detectors.

Yet another aspect of the invention is directed towards an opticalsystem for detecting anomalies of a sample, comprising a sourcesupplying a radiation beam; a switch that causes the radiation beam fromthe source to be transmitted towards the sample surface alternatelyalong a first path and a second path; a detector and means for receivingscattered radiation from the sample surface and originating from thebeam along the first and second paths and for focusing the scatteredradiation to said detector.

Another aspect of the invention is directed towards an optical systemfor detecting anomalies of a sample, comprising means for directing atleast one beam of radiation along a path onto a spot on a surface of thesample; a first detector and means for receiving scattered radiationfrom the sample surface and originating from the at least one beam andfor focusing the scattered radiation to said first detector for sensinganomalies. The system further comprises a second, position sensitive,detector detecting a specular reflection of said at least one beam inorder to detect any change in height of the surface at a spot; and meansfor altering the path of the at least one beam in response to thedetected change in height of the surface of the spot to reduce positionerror of the spot caused by change in height of the surface of the spot.

Still another aspect of the invention is directed towards an opticalsystem for detecting anomalies of a sample, comprising means fordirecting at least one beam of radiation along a path onto a spot on asurface of the sample; a first detector and means for collectingscattered radiation from the sample surface and originating from the atleast one beam and for conveying the scattered radiation to said firstdetector for sensing anomalies. The system further comprises a spatialfilter between the first detector and the collecting and conveying meansblocking scattered radiation towards the detector except for at leastone area having a wedge shape.

One more aspect of the invention is directed towards an optical methodfor detecting anomalies of a sample, comprising directing a first beamof radiation along a first path onto a surface of the sample; directinga second beam of radiation along a second path onto a sample of thesurface; employing a mirrored surface for receiving scattered radiationfrom the sample surface and originating from the first and second beamsand focusing the scattered radiation to a first detector.

Yet another aspect of the invention is directed towards an opticalmethod for detecting anomalies of a sample, comprising directing a firstbeam of radiation along a first path onto a surface of the sample;directing a second beam of radiation along a second path onto a surfaceof the sample; said first and second beams producing respectively afirst and a second illuminated spot on the sample surface, said firstand second illuminated spots separated by an offset. The method furthercomprises receiving scattered radiation from the first and secondilluminated spots and for focusing the scattered radiation to adetector.

An additional aspect of the invention is directed towards an opticalmethod for detecting anomalies of a sample, comprising supplying a beamof radiation of a first and a second wavelength; converting theradiation beam into a first beam at a first wavelength along a firstpath and a second beam at a second wavelength along a second path, saidtwo beams directed towards a surface of the sample. The method furthercomprises collecting scattered radiation from the sample surface andoriginating from the first and second beams, focusing the collectedscattered radiation to one or more detectors, and detecting radiation atthe first and second wavelengths by means of said detectors.

Yet another aspect of the invention is directed towards an opticalmethod for detecting anomalies of a sample, comprising supplying aradiation beam, switching alternately the radiation beam between a firstand a second path towards a surface of the sample, receiving scatteredradiation from the sample surface and originating from the beam alongthe first and second paths, and focusing the scattered radiation to adetector.

Another aspect of the invention is directed towards an optical methodfor detecting anomalies of a sample, comprising directing at least onebeam of radiation along a path onto a spot on the surface of the sample;collecting scattered radiation from the sample surface and originatingfrom the at least one beam, and focusing the collected scatteredradiation to a first detector, for sensing anomalies. The method furthercomprises detecting a specular reflection of said at least one beam inorder to detect any change in height of the surface at the spot andaltering the path of the at least one beam in response to the detectedchange in height of the surface of the spot to reduce position error ofthe spot caused by change in height of the surface of the spot.

One more aspect of the invention is directed towards an optical methodfor detecting anomalies of a sample, comprising directing at least onebeam of radiation along a path onto a spot on a surface of the sample;collecting scattered radiation from the sample surface and originatingfrom the at least one beam, conveying the scattered radiation to a firstdetector for sensing anomalies, and blocking scattered radiation towardsthe detector except for at least one area having a wedge shape.

Still another aspect of the invention is directed towards an opticalsystem for detecting anomalies of a sample, comprising means fordirecting a beam of radiation along a path at an oblique angle to asurface of the sample; a detector and means including a curved mirroredsurface for collecting scattered radiation from the sample surface andoriginating from the beam and for focusing the scattered radiation tosaid detector.

One more aspect of the invention is directed towards an optical methodfor detecting anomalies of a sample, comprising directing a beam ofradiation along a path at an oblique angle to a surface of the sample;providing a curved mirrored surface to collect scattered radiation fromthe sample surface and originating from the beam, and focusing thescattered radiation from the mirrored surface to a detector to detectanomalies of the sample.

Another aspect of the invention enables the distinction between COPs andparticles on the surface. When the surface is illuminated by aP-polarized beam at an oblique angle of incidence, the radiationscattered by a particle has more energy in directions away from thenormal direction of the surface compared to directions close to thenormal direction to the surface. The radiation scattered by a COP fromoblique incident P-polarized light is more uniform compared to that ofthe particle. Therefore, by detecting radiation scattered in directionsaway from the normal direction to the surface and comparing it toradiation scattered in directions close to the normal direction to thesurface, it is possible to distinguish between COPs and particles.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B and 1C are schematic views of normal or obliqueillumination beams illuminating a surface with a particle thereon usefulfor illustrating the invention.

FIG. 2A is a schematic view of a sample inspection system employing anellipsoidal mirror for illustrating one embodiment of the invention.

FIG. 2B is a schematic view of a sample inspection system employing aparaboloidal mirror to illustrate another embodiment of the invention.

FIG. 3 is an exploded simplified view of a portion of the system of FIG.2A or FIG. 2B to illustrate another aspect of the invention.

FIG. 4 is a schematic view of a sample inspection system employing twodifferent wavelengths for illumination to illustrate yet anotherembodiment of the invention.

FIGS. 5A and 5B are schematic views of sample inspection systemsillustrating two different embodiments employing switches for switchinga radiation beam between a normal illumination path and an obliqueillumination path to illustrate yet another aspect of the invention.

FIG. 6 is a schematic view of a beam illuminating a semiconductor wafersurface to illustrate the effect of a change in height of a wafer on theposition of the spot illuminated by beam.

FIG. 7 is a schematic view of a portion of a sample inspection systeminspecting a semiconductor wafer, employing three lenses, where thedirection of the illumination beam is altered to reduce the error in theposition of the illuminated spot caused by the change in height of thewafer.

FIG. 8 is a schematic view of a portion of a sample inspection systememploying only one lens to compensate for a change in height of thewafer.

FIGS. 9A-9F are schematic views of six different spatial filters usefulfor detecting anomalies of samples.

FIG. 10A is a simplified partially schematic and partiallycross-sectional view of a programmable spatial filter employing a layerof liquid crystal material sandwiched between an electrode and an arrayof electrodes in the shape of sectors of a circle and means for applyinga potential difference across at least one sector in the array and theother electrode, so that the portion of the liquid crystal layeradjacent to the at least one sector is controlled to be radiationtransparent or scattering.

FIG. 10B is a top view of the filter of FIG. 10A.

FIG. 11 is a schematic view of a sample inspection system employing anoblique illumination beam and two detectors for distinguishing betweenCOPs and particles to illustrate another aspect of the invention.

For simplicity in description, identical components are labelled by thesame numerals in this application.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1A is a schematic view of a surface 20 of a sample to be inspectedand an illumination beam 22 directed in a direction normal to surface 20to illuminate the surface and a particle 24 on the surface. Thus, theillumination beam 22 illuminates an area or spot 26 of surface 20 and adetection system (not shown) detects light scattered by particle 24 andby portion or spot 26 of the surface 20. The ratio of the photon fluxreceived by the detector from particle 24 to that from spot 26 indicatesthe sensitivity of the system to particle detection.

If an illumination beam 28 directed at an oblique angle to surface 20 isused to illuminate spot 26′ and particle 24 instead, as shown in FIG.1B, from a comparison between FIGS. 1A and 1B, it will be evident thatthe ratio of the photon flux from the particle 24 to that from theilluminated spot will be greater in the case of the oblique illuminationin FIG. 1B compared to that in FIG. 1A. Therefore, for the samethroughput (spots 26, 26′ having the same area), the sensitivity of theoblique incidence beam in detecting small particles is superior and isthe method of choice in the detection of small particles.

FIG. 1C illustrates an oblique beam 28′ illuminating a surface 30 havinga pit 32 and particle 24′ thereon. As can be seen from FIG. 1C, eventhough the pit 32 is of comparable size to particle 24, it will scattera much smaller amount of photon flux compared to particle 24 fromoblique beam 28′. On the other hand, if the pit 32 and particle 24 areilluminated by a beam such as 22 directed in a direction normal tosurface 30, pit 32 and particle 24 would cause comparable amount ofphoton flux scattering. Almost regardless of the exact shape ororientation of COPs and particles, anomaly detection employing obliqueillumination is much more sensitive to particles than COPs. In the caseof anomaly detection with normal illumination, however, thedifferentiation between particles and COPs is less pronounced.Therefore, by means of a simultaneous, or sequential, comparison offeature signatures due to normal and oblique illumination will revealwhether the feature is a particle or a COP.

Azimuthal collection angle is defined as the angle made by thecollection direction to the direction of oblique illumination whenviewed from the top. By employing oblique illumination, together with ajudicious choice of the azimuthal collection angle, rough films can beinspected with good sensitivity, such as when a spatial filter shown inany of FIGS. 9A-9F, 10A and 10B is used in any one of the embodiments asshown in FIGS. 2A, 2B, 3, 4, 5A and 5B, as explained below. By retainingthe normal illumination beam for anomaly detection, all of theadvantageous attributes of the system described in PCT PatentApplication No. PCT/US96/15354 noted above, are retained, including itsuniform scratch sensitivity and the possibility of adding a bright-fieldchannel as described in PCT Patent Application No. PCT/US97/04134, filedMar. 5, 1997, entitled “Single Laser Bright Field and Dark Field Systemfor Detecting Anomalies of a Sample.”

Scanning a sample surface with oblique and normal illumination beams canbe implemented in a number of ways. FIG. 2A is a schematic view of asample inspection system to illustrate a general set up for implementinganomaly detection using both normal and oblique illumination beams. Aradiation source that provides radiation at one or more wavelengths in awide electromagnetic spectrum (including but not limited to ultraviolet,visible, infrared) may be used, such as a laser 52 providing a laserbeam 54. A lens 56 focuses the beam 54 through a spatial filter 58 andlens 60 collimates the beam and conveys it to a polarizing beamsplitter62. Beamsplitter 62 passes a first polarized component to the normalillumination channel and a second polarized component to the obliqueillumination channel, where the first and second components areorthogonal. In the normal illumination channel 70, the first polarizedcomponent is focused by optics 72 and reflected by mirror 74 towards asample surface 76 a of a semiconductor wafer 76. The radiation scatteredby surface 76 a is collected and focused by an ellipsoidal mirror 78 toa photomultiplier tube 80.

In the oblique illumination channel 90, the second polarized componentis reflected by beamsplitter 62 to a mirror 82 which reflects such beamthrough a half-wave plate 84 and focused by optics 86 to surface 76 a.Radiation originating from the oblique illumination beam in the obliquechannel 90 and scattered by surface 76 a is collected by an ellipsoidalmirror and focused to photomultiplier tube 80. Photomultiplier tube 80has a pinhole entrance 80 a. The pinhole 80 a and the illuminated spot(from the normal and oblique illumination channels on surface 76 a) arepreferably at the foci of the ellipsoidal mirror 78.

Wafer 76 is rotated by a motor 92 which is also moved linearly bytransducer 94, and both movements are controlled by a controller 96, sothat the normal and oblique illumination beams in channels 70 and 90scan surface 76 a along a spiral scan to cover the entire surface.

Instead of using an ellipsoidal mirror to collect the light scattered bysurface 76 a, it is also possible to use other curved mirrors, such as aparaboloidal mirror 78′ as shown in system 100 of FIG. 2B. Theparaboloidal mirror 78′ collimates the scattered radiation from surface76 a into a collimated beam 102 and the collimated beam 102 is thenfocused by an objective 104 and through an analyzer 98 to thephotomultiplier tube 80. Aside from such difference, the sampleinspection system 100 is exactly the same as system 50 of FIG. 2A.Curved mirrored surfaces having shapes other than ellipsoidal orparaboloidal shapes may also be used; preferably, each of such curvedmirrored surfaces has an axis of symmetry substantially coaxial with thepath of the normal illumination path, and defines an input aperture forreceiving scattered radiation. All such variations are within the scopeof the invention. For simplicity, the motor, transducer and control formoving the semiconductor wafer has been omitted from FIG. 2B and fromFIGS. 4, 5A, 5B described below.

The general arrangements shown in FIGS. 2A and 2B can be implemented indifferent embodiments. Thus, in one arrangement referred to below as the“GO and RETURN” option, a half-wave plate (not shown) is added betweenlaser 52 and lens 56 in FIGS. 2A and 2B so that the polarization of thelight reaching the beamsplitter 62 can be switched between P and S.Thus, during the Go cycle, the beamsplitter 62 passes radiation onlyinto the normal channel 70 and no radiation will be directed towards theoblique channel 90. Conversely, during the RETURN cycle, beamsplitter 62passes radiation only into the oblique channel 90 and no radiation willbe directed through the normal channel 70. During the GO cycle, only thenormal illumination beam 70 is in operation, so that the light collectedby detector 80 is recorded as that from normal illumination. This isperformed for the entire surface 76 a where motor 92, transducer 94 andcontrol 96 are operated so that the normal illumination beam 70 scansthe entire surface 76 a along a spiral scan path.

After the surface 76 a has been scanned using normal illumination, thehalf-wave plate between laser 52 and lens 56 causes radiation from laser52 to be directed only along the oblique channel 90 and the scanningsequence by means of motor 92, transducer 94 and control 96 is reversedand data at detector 80 is recorded in a RETURN cycle. As long as theforward scan in the Go cycle and the reverse scan in the RETURN cycleare exactly registered, the data set collected during the GO cycle andthat collected during the return cycle may be compared to provideinformation concerning the nature of the defects detected. Instead ofusing a half-wave plate and a polarizing beamsplitter as in FIG. 2A, theabove-described operation may also be performed by replacing suchcomponents with a removable mirror placed in the position ofbeamsplitter 62. If the mirror is not present, the radiation beam fromlaser 52 is directed along the normal channel 70. When the mirror ispresent, the beam is then directed along the oblique channel 90. Suchmirror should be accurately positioned to ensure exact registration ofthe two scans during the GO and RETURN cycles. While simple, theabove-described GO and RETURN option requires extra time expended in theRETURN cycle.

The normal illumination beam 70 illuminates a spot on surface 76 a. Theoblique illumination beam 90 also illuminates a spot on the surface 76a. In order for comparison of data collected during the two cycles to bemeaningful, the two illuminated spots should have the same shape. Thus,if beam 90 has a circular cross-section, it would illuminate anelliptical spot on the surface. In one embodiment, focusing optics 72comprises a cylindrical lens so that beam 70 has an ellipticalcross-section and illuminates also an elliptical spot on surface 76 a.

To avoid having to scan surface 76 a twice, it is possible tointentionally introduce a small offset between the illuminated spot 70 afrom normal illumination beam 70 (referred to herein as “normalillumination spot” for simplicity) and the illuminated spot 90 a fromoblique illumination beam 90 (referred to herein as “obliqueillumination spot” for simplicity) as illustrated in FIG. 3. FIG. 3 isan enlarged view of surface 76 a and the normal and oblique illuminationbeams 70, 90 to illustrate an offset 120 between the normal and obliqueillumination spots 70 a, 90 a. In reference to FIGS. 2A, 2B, radiationscattered from the two spots 70 a, 90 a would be detected at differenttimes and would be distinguished.

The method illustrated in FIG. 3 causes a reduction in system resolutionand increased background scattering due to the presence of both spots.In other words, in order that radiation scattered from both spotsseparated by an offset would be focused through pinhole 80 a, thepinhole should be somewhat enlarged in the direction of the offset. As aconsequence, detector 80 will sense an increased background scatteringdue to the enlargement of the pinhole 80 a. Since the background is dueto both beams whereas the particle scattered radiation is due to one orthe other spot, the signal-to-noise ratio is decreased. Preferably, theoffset is not greater than three times the spatial extent, or less thanthe spatial extent, of the point spread function of either the normal oroblique illumination beam. The method illustrated in FIG. 3, however, isadvantageous since throughput is not adversely affected compared to thatdescribed in PCT Application No. PCT/US96/15354 and the Censor ANSseries of inspection systems from KLA-Tencor Corporation of San Jose,Calif., the assignee of this application.

FIG. 4 is a schematic view of a sample inspection system employing anormal illumination beam comprising radiation at a first wavelength λ₁and an oblique illumination beam of radiation of wavelength λ₂ toillustrate another embodiment of the invention. The laser 52 of FIGS.2A, 2B may supply radiation at only one wavelength, such as 488 nm ofargon. Laser 52′ of FIG. 4 supplies radiation at at least two differentwavelengths in beam 54′, such as at 488 and 514 nm, instead of radiationof only one wavelength, Such beam is split by a dichroic beamsplitter162 into a first beam at a first wavelength λ₁ (488 nm) and a secondbeam of wavelength λ₂ (514 nm), by passing radiation at wavelength λ₁and reflecting radiation at wavelength λ₂, for example. After beingfocused by optics 72, beam 70′ at wavelength λ₁ is reflected by mirror74 towards surface 76 a as the normal illumination beam. The reflectedradiation of wavelength λ₂ at beamsplitter 162 is further reflected bymirror 82 and focused by optics 86 as the oblique illumination beam 90′to illuminate the surface. The optics in both the normal and obliqueillumination paths are such that the normal and oblique illuminatedspots substantially overlap with no offset there between. The radiationscattered by surface 76 a retains the wavelength characteristics of thebeams from which the radiation originate, so that the radiationscattered by the surface originating from normal illumination beam 70′can be separated from radiation scattered by the surface originatingfrom oblique illumination beam 90′. Radiation scattered by surface 76 ais again collected and focused by an ellipsoidal mirror 78 through apinhole 164 a of a spatial filter 164 to a dichroic beamsplitter 166. Inthe embodiment of FIG. 4, beamsplitter 166 passes the scatteredradiation at wavelength λ₁ to detector 80(1) through a lens 168.Dichroic beamsplitter 166 reflects scattered radiation at wavelength λ₂through a lens 170 to photomultiplier tube 80(2). Again, the mechanismfor causing the wafer to rotate along a spiral path has been omittedfrom FIG. 4 for simplicity.

Instead of using a laser that provides radiation at a single wavelength,the laser source 52′ should provide radiation at two distinctwavelengths. A commercially available multi-line laser source that maybe used is the 2214-65-ML manufactured by Uniphase, San Jose, Calif. Theamplitude stability of this laser at any given wavelength is around3.5%. If such a laser is used, the scheme in FIG. 4 will be useful forapplications such as bare silicon inspection but may have diminishedparticle detection sensitivity when used to scan rough films.

Yet another option for implementing the arrangements generally shown inFIGS. 2A and 2B is illustrated in FIGS. 5A and 5B. In such option, aradiation beam is switched between the normal and oblique illuminationchannels at a higher frequency than the data collection rate so that thedata collected due to scattering from the normal illumination beam maybe distinguished from data collected from scattering due to the obliqueillumination channel. Thus as shown in FIG. 5A, an electro-opticmodulator (e.g. a Pockels cell) 182 is placed between laser 52 andbeamsplitter 62 to modulate the radiation beam 54 at the half-wavevoltage. This results in the beam being either transmitted or reflectedby the polarizing beamsplitter 62 at the drive frequency of modulator182 as controlled by a control 184.

The electro-optic modulator may be replaced by a Bragg modulator 192 asshown in FIG. 5B, which may be turned on and off at a high frequency ascontrolled. Modulator 192 is powered by block 193 at frequency ω_(b).This block is turned on and off at a frequency ω_(m). In the offcondition, a zero order beam 194 a passes through the Bragg modulator192, and becomes the normal illumination beam reflected to surface 76 aby mirror 74. In the on condition, cell 192 generates a deflected firstorder beam 194 b, which is reflected by mirrors 196, 82 to surface 76 a.However, even though most of the energy from cell 192 is directed to theoblique first order beam, a weak zero order normal illumination beam isstill maintained, so that the arrangement in FIG. 5B is not as good asthat in FIG. 5A.

Preferably, the electro-optic modulator of FIG. 5A and the Braggmodulator of FIG. 5B are operated at a frequency higher than the datarate, and preferably, at a frequency at least about 3 or 5 times thedata rate of tube 80. As in FIG. 4, the optics in both the normal andoblique illumination paths of FIGS. 5A, 5B are such that the normal andoblique illuminated spots substantially overlap with no offset therebetween. The arrangements in FIGS. 2A, 2B, 4, 5A, 5B are advantageous inthat the same radiation collector 78 and detector 80 are used fordetecting scattered light originating from the normal illumination beamas well as from the oblique illumination beam. Furthermore, by employinga curved surface that collects radiation that is scattered within therange of at least 25 to 70° from a normal direction to surface 76 a andfocusing the collected radiation to the detector, the arrangements ofFIG. 2A, 2B, 4, 5A, 5B maximize the sensitivity of detection.

In contrast to arrangements where multiple detectors are placed atdifferent azimuthal collection angles relative to the obliqueillumination beam, the arrangements of FIG. 2A, 2B has superiorsensitivity and is simpler in arrangement and operation, since there isno need to synchronize or correlate the different detection channelsthat would be required in a multiple detector arrangement. Theellipsoidal mirror 78 collects radiation scattered within the range ofat least 25 to 70° from the normal direction to the surface whichaccounts for most of the radiation that is scattered by surface 76 afrom an oblique illumination beam, and that contains information usefulfor particle and COPs detection.

The three dimensional intensity distribution of scattered radiation fromsmall particles on the surface when the surface is illuminated by aP-polarized illumination beam at or near a grazing angle to the surfacehas the shape of a toroid. In the case of large particles, higherscattered intensity is detected in the forward direction compared toother directions. For this reason, the curved mirror collectors of FIGS.2A, 2B, 4, 5A, 5B are particularly advantageous for collecting thescattered radiation from small and large particles and directing thescattered radiation towards a detector. In the case of normalillumination, however, the intensity distribution of radiation scatteredfrom small particles on surfaces is in the shape of a sphere. Thecollectors in FIGS. 2A, 2B, 4, 5A, 5B are also advantageous forcollecting such scattered radiation. Preferably, the illumination angleof beam 90 is within the range of 45 to 85° from a normal direction tothe sample surface, and preferably at 70 or 75°, which is close to theprincipal angle of silicon at 488 and 514 nm, and would allow the beampassage to be unhindered by the walls of the collector. By operating atthis shallow angle, the particle photon flux is enhanced as illustratedin FIGS. 1A and 1B and the discrimination against the pits issubstantial.

Beam Position Correction

A prerequisite for the comparison of signals generated by two detectionchannels for a given defect is the ability to place the two spots on thesame location. In general, semiconductor wafers or other sample surfacesare not completely flat, nor do they have the same thickness. Suchimperfections are of little concern for anomaly detection employing anormal incidence beam, as long as the wafer surface remains within thedepth of focus. In the case of the oblique illumination beam, however,wafer-height variation will cause the beam position and hence theposition of the illuminated spot to be incorrect. In FIG. 6, θ is theoblique incidence angle between the bean and a normal direction N to thewafer surface. Thus, as shown in FIG. 6, if the height of the wafersurface moves from the dotted line position 76 a′ to the solid lineposition 76 a which is higher than the dotted line position by theheight h, then the position of the illuminated spot on the wafer surfacewill be off by an error of w given by h.tan θ. One possible solution isto detect the change in height of the wafer at the illuminated spot andmove the wafer in order to maintain the wafer at a constant height atthe illuminated spot, as described in U.S. Pat. No. 5,530,550. In theembodiment described above, the wafer is rotated and translated to movealong a spiral scan path so that it may be difficult to also correct thewafer height by moving the wafer while it is being rotated along suchpath. Another alternative is to move the light source and the detectorwhen the height of the wafer changes so as to maintain a constant heightbetween the light source and the detector on the one hand and the wafersurface at the illuminated spot on the other. This is obviouslycumbersome and may be impractical. Another aspect of the invention isbased on the observation that, by changing the direction of theillumination beam in response to a detected change in wafer height, itis possible to compensate for the change in wafer height to reduce beamposition error caused thereby.

One scheme for implementing the above aspect is illustrated in FIG. 7.As shown in system 200 of FIG. 7, an illumination beam is reflected by amirror 202 and focused through three lenses L₁, L₂, L₃ to the wafersurface 204 a. The positions of the lenses are set in order to focus anoblique illumination beam 70″ to wafer surface 204 a in dotted line inFIG. 7. Then a quad cell (or other type of position sensitive detector)206 is positioned so that the specular reflection 70 a″ of the beam 70″from surface 204 reaches the cell at the null or zero position 206 a ofthe cell. As the wafer surface moves from position 204 a to 204 b shownin solid line in FIG. 7, such change in height of the wafer causes thespecular reflection to move to position 70 b″, so that it reaches thecell 206 at a position on the cell offset from the null position 206 a.Detector 206 may be constructed in the same manner as that described inU.S. Pat. No. 5,530,550. A position error signal output from detector206 indicating the deviation from the null position in two orthogonaldirections is sent by cell 206 to a control 208 which generates an errorsignal to a transducer 210 for rotating the mirror 202 so that thespecular reflection 70 b″ also reaches the cell at the null position 206a. In other words, the direction of the illumination beam is altereduntil the specular reflection reaches the cell at null position, atwhich point control 208 applies no error signal to the transducer 210.

Instead of using three lenses, it is possible to employ a single lens asshown in FIG. 8, except that the correct placement of the illuminatedspot on the wafer corresponds not to a null in the position sensingsignal from the position sensitive detector, but corresponds to anoutput of the detector reduced by ½. This approach is shown in FIG. 8.Thus, controller 252 divides by 2 the amplitude of the position sensingsignal at the output of quad cell detector 254 to derive a quotientsignal and applies the quotient signal to transducer 210. The transducer210 rotates the mirror by an amount proportional to the amplitude of thequotient signal. The new position of the specular reflection correspondsto the correct location of the spot. The new error signal is now the newreference.

The above described feature of reducing beam position error of theoblique illumination beam in reference to FIGS. 7 and 8 may be used inconjunction with any one of the inspection systems of FIGS. 2A, 2B, 3,4, 5A and 5B, although only the quad cell (206 or 254) is shown in thesefigures.

Spatial Filter

In reference to the embodiments of FIGS. 2A, 2B, 4, 5A and 5B, it isnoted that the radiation collection and detection schemes in suchembodiments retain the information concerning the direction ofscattering of the radiation from surface 76 a relative to the obliqueillumination channel 90 or 90′. This can be exploited for someapplications such as rough surface inspection. This can be done byemploying a spatial filter which blocks the scattered radiationcollected by the curved mirrored surface towards the detector except forat least one area have a wedge shape. With respect to the normalillumination channel, there is no directional information since both theillumination and scattering are symmetrical about a normal to thesurface. In other words, if the normal illumination channel is omittedin the embodiments of FIGS. 2A, 2B, 4, 5A and 5B, the curved mirroredcollector 78 or 78′ advantageously collects most of the radiationscattered within the toroidal intensity distribution caused by particlescattering to provide an inspection tool of high particle sensitivity.At the same time, the use of a curved mirrored collector retains thedirectional scattering information, where such information can beretrieved by employing a spatial filter as described below.

FIGS. 9A-9F illustrate six different embodiments of such spatial filtersin the shape of butterflies each with two wings. The dark or shadedareas (wings) in these figures represent areas that are opaque to orscatters radiation, and the white or unshaded areas represent areas thattransmit such radiation. The size(s) of the radiation transmissive(white or unshaded) area(s) are determined in each of the filters inFIGS. 9A-9F by the wedge angle α. Thus, in FIG. 9A, the wedge angle is10°, whereas in FIG. 9B, it is 20°.

Thus, if the filter in FIG. 9B is placed at position 300 of FIG. 2A, 2B,4, 5A or 5B where the 20° wedge-shaped area of radiation collection iscentered at approximately 90° and 270° azimuthal collection anglesrelative to the oblique illumination direction, this has the effect ofgenerating a combined output from two detectors, each with a collectionangle of 20°, one detector placed to collect radiation between 80 to100° azimuthal angles as in U.S. Pat. No. 4,898,471, and the otherdetector to collect radiation between 260 and 280° azimuthal angles. Thedetection scheme of U.S. Pat. No. 4,898,471 can be simulated by blockingout also the wedge area between 260 and 280 azimuthal angles. Thearrangement of this application has the advantage over U.S. Pat. No.4,898,471 of higher sensitivity since more of the scattered radiation iscollected than in such patent, by means of the curved mirror collector78, 78′. Furthermore, the azimuthal collection angle can be dynamicallychanged by programming the filter at position 300 in FIGS. 2A, 2B, 4,5A, 5B without having to move any detectors, as described below.

It is possible to enlarge or reduce the solid angle of collection of thedetector by changing α. It is also possible to alter the azimuthalangles of the wedge areas. These can be accomplished by having ready athand a number of different filters with different wedge angles such asthose shown in FIGS. 9A-9F, as well as filters with other wedge shapedradiation transmissive areas, and picking the desired filter and thedesired position of the filter for use at position 300 in FIGS. 2A, 2B,4, 5A, 5B. The spatial filters in FIGS. 9A-9E are all in the shape ofbutterflies with two wings, where the wings are opaque to, or scatter,radiation and the spaces between the wings transmit radiation betweenthe mirrored surfaces and detector 80. In some applications, however, itmay be desirable to employ a spatial filter of the shape shown in FIG.9F having a single radiation transmissive wedge-shaped area. Obviously,spatial filters having any number of wedge-shaped areas that areradiation transmissive dispersed around a center at various differentangles may also be used and are within the scope of the invention.

Instead of storing a number of filters having different wedge angles,different numbers of wedges and distributed in various configurations,it is possible to employ a programmable spatial filter where the opaqueor scattering and transparent or transmissive areas may be altered. Forexample, the spatial filter may be constructed using corrugated materialwhere the wedge angle α can be reduced by flattening the corrugatedmaterial. Or, two or more filters such as those in FIGS. 9A-9F may besuperimposed upon one another to alter the opaque or scattering andtransparent or transmissive areas.

Alternatively, a liquid crystal spatial filter may be advantageouslyused, one embodiment of which is shown in FIGS. 10A and 10B. A liquidcrystal material can be made radiation transmissive or scattering bychanging an electrical potential applied across the layer. The liquidcrystal layer may be placed between a circular electrode 352 and anelectrode array 354 in the shape of n sectors of a circle arrangedaround a center 356, where n is a positive integer. The sectors areshown in FIG. 10B which is a top view of one embodiment of filter 350 inFIG. 10A. Adjacent electrode sectors 354(i) and 354(i+1), i ranging from1 to n−1, are electrically insulated from each other.

Therefore, by applying appropriate electrical potentials across one ormore of the sector electrodes 354(i), where (i) ranges from 1 to n, onone side, and electrode 352 on the other side, by means of voltagecontrol 360, it is possible to programmably change the wedge angle α byincrements equal to the wedge angle β of each of the sector electrodes354(1) through 354(n). By applying the potentials across electrode 352and the appropriate sector electrodes, it is also possible to achievefilters having different numbers of radiation transmissive wedge-shapedareas disposed in different configurations around center 356, again withthe constraint of the value of β. To simplify the drawings, theelectrical connection between the voltage control 360 and only one ofthe sector electrodes is shown in FIGS. 10A and 10B. Instead of being inthe shape of sectors of a circle, electrodes 354 can also be in theshape of triangles. Where electrodes 354 are shaped as isoscelestriangles, the array of electrodes 354 arranged around center 356 hasthe shape of a polygon. Still other shapes for the array 354 arepossible.

If the wedge angle β is chosen to be too small, this means that aninordinate amount of space must be devoted to the separation betweenadjacent sector electrodes to avoid electrical shorting. Too large avalue for β means that the wedge angle α can only be changed by largeincrements. Preferably β is at least about 5°.

For the normal illumination beam, the polarization state of the beamdoes not, to first order, affect detection. For the oblique illuminationbeam, the polarization state of the beam can significantly affectdetection sensitivity. Thus, for rough film inspection, it may bedesirable to employ S polarized radiation, whereas for smooth surfaceinspection, S or P polarized radiation may be preferable. After thescattered radiation from the sample surface originating from each of thetwo channels have been detected, the results may be compared to yieldinformation for distinguishing between particles and COPS. For example,the intensity of the scattered radiation originating from the obliquechannel (e.g., in ppm) may be plotted against that originating from thenormal channel, and the plot is analyzed. Or a ratio between the twointensities is obtained for each of one or more locations on the samplesurface. Such operations may be performed by a processor 400 in FIGS.2A, 2B, 4, 5A, 5B.

As noted above in connection with FIG. 1C, a pit 32 of comparable sizeto a particle 24 will scatter a smaller amount of photon flux comparedto particle 24 from an oblique beam 28′. Moreover, if the obliqueincidence bean is P-polarized, the scattering caused by the particle ismuch stronger in directions at large angles to the normal direction tothe surface compared to the scattering in directions close to the normaldirection. This is not the case with a COP whose scattering pattern foran oblique incident P-polarized beam is more uniform inthree-dimensional space. This feature can be exploited as illustrated inFIG. 11.

In reference to FIGS. 2A and 11, the sample inspection system 500 ofFIG. 11 differs from system 50 of FIG. 2A in that an additional detector502 is employed with its corresponding pinhole 504. Direction 510 isnormal to the surface 76 a of the wafer 76. The radiation scattered indirections close to the normal direction 510 are reflected by a mirror512 through the pinhole 504 to a photomultiplier tube 502 for detection.The radiation that is scattered by surface 76 a in directions away fromthe normal direction 510 are collected by mirror 78 and focused topinhole 80 a and photomultiplier tube 80. Thus, detector 80 detectsradiation scattered by surface 76 a in directions at large angles to thenormal direction 510 whereas detector 502 detects radiation scattered bythe surface along directions close to the normal direction 510.

For the purpose of distinguished particles and COPs, the obliqueillumination beam in the oblique channel 90 is preferably P-polarized.In such event, a particle on surface 76 a illuminated by the obliqueillumination beam will scatter radiation in a three-dimensional patternsimilar to a toroid, which is relatively devoid of energy in a normaldirection 510 and in directions close to the normal direction. A COP, onthe other hand, would scatter such beam in a more uniform manner inthree-dimensional space. Therefore, if the signal detected by detector502 differs by a large factor from that detected by detector 80, theanomaly on surface 76 a is more likely to be a particle, whereas if thesignals detected by the two detectors differ by a smaller factor theanomaly present on surface 76 a is more likely to be a COP.

The P-polarized oblique illumination beam in channel 90 may be providedby a laser 52 in the same manner as that described above in reference toFIG. 2A. The S-polarized beam reflected towards mirror 82 by polarizingbeam splitter 62 may be altered into a P-polarized beam by a half-waveplate 84. Since the illumination beam in the normal channel is not usedin system 500, it may simply be blocked (not shown in FIG. 11). Acomparison of the outputs of the two detectors 80, 502 may be performedby a processor 400. Mirror 78 may be ellipsoidal in shape, orparaboloidal in shape (in which case an additional objective similar toobjective 104 of FIG. 2B is also employed) or may have other suitableshapes.

While the invention has been described by reference to a normal and anoblique illumination beam, it will be understood that the normalillumination beam may be replaced by one that is not exactly normal tothe surface, while retaining most of the advantages of the inventiondescribed above. Thus, such beam may be at a small angle to the normaldirection, where the small angle is no more than 10° to the normaldirection.

While the invention has been described above by reference to variousembodiments, it will be understood that changes and modifications may bemade without departing from the scope of the invention, which is to bedefined only by the appended claims and their equivalents. For example,while only two illuminating beams or paths are shown in FIGS. 2A, 2B, 4,5A, 5B, it will be understood that three or more illuminating beams orpaths may be employed and are within the scope of the invention.

1. An optical system for detecting anomalies of a sample, comprising:optics directing a beam of radiation along a first path at an obliqueangle of incidence onto a surface of the sample; at least two detectorscomprising a first detector located to detect light scattered by thesurface within a first range of collection angles from a normaldirection to the surface and a second detector comprising a curvedmirrored surface and located to detect light scattered by the surfacewithin a second range of collection angles from the normal direction,said second range being different from the first range; and a devicecomparing outputs of the two detectors to distinguish between a particleand a COP on the surface.
 2. The system of claim 1, wherein thecollection angles of said first range are smaller than the collectionangles of said second range, said first detector including at least onelens for collecting light to be detected.
 3. The system of claim 1, saidmirrored surface being substantially ellipsoidal in shape.
 4. A methodfor detecting anomalies of a sample, comprising: directing a beam ofradiation along a first path at an oblique angle of incidence onto asurface of the sample; detecting light scattered by the surface within afirst and a second range of collection angles from a normal direction tothe surface, said second range being different from the first range,wherein light within the first range is detected by means of a firstdetector and light within the second range is collected by a curvedmirrored surface and detected by means of a second detector; andcomparing outputs of the two detectors to distinguish between a particleand a COP on the surface.